Image forming apparatus

ABSTRACT

An image forming apparatus includes an image forming unit configured to from a measurement image on a recording paper by a color material, a fixing unit configured to fix the measurement image onto the recording paper by heating, a measurement unit configured to measure the measurement image fixed onto the recording paper downstream of the fixing unit in a conveyance direction of the recording paper, and a control unit configured to perform control such that a period of time from when the recording paper passes through the fixing unit until the measurement unit measures color of the measurement image is longer than a period of time from when the recording paper passes through the fixing unit until the measurement unit measures density of the measurement image.

BACKGROUND OF THE INVENTION

The present application is a Continuation of U.S. patent applicationSer. No. 13/648,182 filed Oct. 9, 2012, which claims the benefit ofpriority from Japanese Patent Application No. 2011-226024 filed Oct. 13,2011. U.S. patent application Ser. No. 13/648,182 and Japanese PatentApplication No. 2011-226024, are hereby incorporated by reference hereinin their entirety.

FIELD OF THE INVENTION

The present invention relates to an image forming apparatus having acolorimetric function.

DESCRIPTION OF THE RELATED ART

In an image forming apparatus, the quality of an image is determinedbased on graininess, in-plane uniformity, character quality, and colorreproducibility (including color stability). In the recent spreading ofthe multi-color image forming apparatus, the color reproducibility issometimes referred to as the most material factor for determining thequality of an image.

Each person has a memory of colors (e.g., specifically, colors of humanskin, blue sky, and metal) he expects based on his experience. Theperson will have an uncomfortable feeling when seeing a color beyond apermissible range of the color he expects. Such colors are called“memory colors”. The reproducibility of the memory colors are oftenexpected when photographs are output.

A demand for a good color reproducibility (including color stability) isincreasing with respect to the image forming apparatus. For example, inaddition to the photo-images, there are office users who haveuncomfortable feeling of difference in colors between a document imageon a monitor and an actual document, and graphic art users for whom thecolor reproducibility of a computer-generated (CG) image is of paramountimportance.

To satisfy the good color reproducibility demanded by the users, forexample, Japanese Patent Application Laid-Open No. 2004-086013 discussesan image forming apparatus for scanning a measurement image (i.e., apatch image) formed on a recording paper by using a color sensorprovided in a conveyance path for conveying the recording paper.

In the image forming apparatus, the measurement image is formed on therecording paper by using toners, and a scanning result of themeasurement image measured by the color sensor is fed back to processingconditions such as an amount of exposure and developing bias, therebyenabling reproduction of density, gradation, and a tint to some extent.

However, in the image forming apparatus discussed in Japanese PatentApplication Laid-Open No. 2004-086013, the color sensor is disposed inthe conveyance path in the vicinity of a fixing device, and chromaticityof the measurement image as a measuring object varies according to atemperature. This phenomenon is called “thermochromism”. The“thermochromism” is induced such that a molecular structure forming acolor material such as toner and ink is changed according to “heat”.Note, in the description below, the word “chromaticity” is just beingused to express color. A particular chromaticity value may be expressedin the L*a*b color space. Other color spaces may be used without goingbeyond the scope and spirit of the invention as recited in the claims.Also, the “chromaticity value” is equivalent to the “color value”.

In order to measure a color of the measurement image within the imageforming apparatus, the color measurement is to be performed after thecolor material is placed on the recording paper and in a state where thecolor materials are mixed on the recording paper. In the image formingapparatus using inks as color materials, the color is required to bemeasured after the color materials are dried by heat by using a dryingdevice. In the image forming apparatus using toners as the colormaterials, the color is required to be measured after the toners areheated and fused to be mixed by a fixing device. Therefore, the colorsensor needs to be placed downstream of the drying device and the fixingdevice in a conveyance direction for conveying a recording paper.

On the other hand, in order to form the image forming apparatus in acompact size, a length of the conveyance path from the drying device andthe fixing device to the color sensor needs to be as short as possible.Therefore, the recording paper and the color materials heated by thedrying device and the fixing device are conveyed to the color sensorwithout being cooled to a room temperature. A temperature of therecording paper becomes higher than the room temperature due to atemperature rise in members within the image forming apparatus such as aconveyance guide of the recording paper or an atmospheric temperaturerise within the image forming apparatus.

As described above, in the image forming apparatus equipped with thecolor sensor therein, a colorimetric measurement result which isdifferent from the chromaticity under normal environment (i.e., underroom-temperature environment) may be obtained due to an adverse effectof the thermochromism.

SUMMARY OF THE INVENTION

An example of the present invention is directed to an image formingapparatus capable of precisely detecting chromaticity (color) of ameasurement image by decreasing a chromaticity (color) variation causeddue to an adverse effect of thermochromism.

According to an aspect of the present invention, an image formingapparatus includes an image forming unit configured to form ameasurement image on a recording paper by using a color material, afixing unit configured to fix the measurement image onto the recordingpaper by heating the measurement image, a measurement unit configured tomeasure the measurement image fixed on the recording paper downstream ofthe fixing unit in a conveyance direction for conveying the recordingpaper, and a control unit configured to perform control such that aperiod of time from when the recording paper passes through the fixingunit until the measurement unit measures color of the measurement imagebecomes longer than a period of time from when the recording paperpasses through the fixing unit until the measurement unit measuresdensity of the measurement image.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 is a cross sectional view illustrating a configuration of animage forming apparatus according to an exemplary embodiment of thepresent invention.

FIG. 2 illustrates a configuration of a color sensor.

FIG. 3 is a block diagram illustrating a system configuration of theimage forming apparatus.

FIG. 4 is a schematic view illustrating a color management environment.

FIG. 5 illustrates a trend of a chromaticity variation per colormaterial.

FIG. 6 illustrates a trend of density variation per color material.

FIGS. 7A, 7B, and 7C each illustrate spectral reflectance data indifferent temperatures when a color of a magenta patch image is measuredby a color sensor.

FIGS. 8A and 8B illustrate a filter sensitivity characteristic to beused in density arithmetic processing.

FIG. 9 is a flow chart illustrating an operation of the image formingapparatus.

FIG. 10 is a flow chart illustrating an operation of a maximum densityadjustment.

FIG. 11 is a flow chart illustrating an operation of a gradationadjustment.

FIG. 12 is a flow chart illustrating an operation of multi-colorcorrection processing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

In the exemplary embodiment of the present invention, a solution of theabove described problem is described below employing anelectrophotographic laser beam printer as an example. Anelectrophotographic method is employed here as an example of an imageforming method. However, the exemplary embodiment of the presentinvention is also applicable to an ink jet method and a sublimationmethod. That is, the exemplary embodiment of the present invention iseffective in the image forming apparatus in which the thermochromismphenomenon can occur. In the thermochromism phenomenon, the chromaticityof a measuring object varies according to a temperature. The ink jetmethod uses an image forming unit for forming an image on a recordingpaper by discharging ink and a fixing unit (i.e., a drying unit) fordrying the ink.

FIG. 1 is a cross sectional view illustrating a configuration of animage forming apparatus 100. The image forming apparatus 100 includes ahousing 101. The housing 101 includes mechanisms for constituting anengine unit and a control board containing unit 104. The control boardcontaining unit 104 includes an engine control unit 102 configured tocontrol print process processing (e.g., paper feeding processing)performed by each mechanism and a printer controller 103.

As illustrated in FIG. 1, the engine unit is provided with four stations120, 121, 122, and 123 corresponding to colors of yellow (Y), magenta(M), cyan (C), and black (K), respectively. The stations 120, 121, 122,and 123 are image forming units for forming an image by transferringtoners onto a recording paper 110. The colors of yellow, magenta, cyan,and black are abbreviated to Y, M, C, and K, respectively. Each of thestations 120, 121, 122, and 123 is made of almost the same parts. Aphotosensitive drum 105 of each station is a kind of an image carrierand is uniformly charged with a surface potential by a correspondingprimary charging device 111. Each photosensitive drum 105 is providedwith a latent image formed thereon by laser light output from acorresponding laser 108. Each development unit 112 forms a toner imageby developing the latent image by using a color material (i.e., toner).The toner image (i.e., a visible image) is transferred onto anintermediate transfer member 106. The visible image formed on theintermediate transfer member 106 is further transferred onto a recordingpaper 110 conveyed from any one of storage units 113 by using a transferroller pair 114.

A fixing process mechanism according to the present exemplary embodimentincludes a primary fixing device 150 and a secondary fixing device 160for heating and pressurizing thus transferred toner image onto therecording paper 110 to be fixed to the recording paper 110. The primaryfixing device 150 includes a fixing roller 151 for heating the recordingpaper 110, a pressure belt 152 for causing the recording paper 110 to bein pressure-contact with the fixing roller 151, and a first post fixingsensor 153 for detecting a completion of fixing of the toner image. Eachof the rollers is a hollow roller and includes a heater therein.

A secondary fixing device 160 is disposed downstream of the primaryfixing device 150 in a conveyance direction of the recording paper 110.The secondary fixing device 160 adds gloss to the toner image fixed bythe primary fixing device 150 onto the recording paper 110 and securesfixity of the toner image. The secondary fixing device 160 alsoincludes, as similar to the primary fixing device 150, a fixing roller161, a pressure roller 162, and a second post fixing sensor 163.According to a type of the recording paper 110, the recording paper 110is not required to be passed through the secondary fixing device 160. Inthis case, the recording paper 110 passes through a conveyance path 130without going through the secondary fixing device 160 for the purpose ofsaving energy consumption.

For example, in a case where a setting is made so as to add more glossonto the recording paper 110 or in a case where more heating energy isrequired in fixing the toner image onto the recording paper 110 which isthicker than the usual paper (i.e., a thick paper), the recording paper110 having passed through the primary fixing device 150 is conveyed tothe secondary fixing device 160. On the other hand, in a case where therecording paper 110 is a plain paper or a thin paper and in a case wherethe setting to add more gloss to the toner image is not made, therecording paper 110 is conveyed through the conveyance path 130 whichdetours around the secondary fixing device 160. A flapper 131 controlswhether the recording paper 110 is to be conveyed to the secondaryfixing device 160 or to be conveyed by detouring around the secondaryfixing device 160.

A conveyance path switching flapper 132 serves as a leading member forleading the recording paper 110 to a paper discharge path 135 or leadingthe recording paper 110 to a paper discharge path 139 connected to theoutside. The leading edge of the recording paper 110 guided to the paperdischarge path 135 passes through a reversal sensor 137 to be conveyedto a reversal unit 136. When the reversal sensor 137 detects thetrailing edge of the recording paper 110, the conveyance direction ofthe recording paper 110 is switched. The conveyance path switchingflapper 133 serves as a leading member for leading the recording paper110 to either one of the paper discharge path 135 and a conveyance path138 for forming an image on both sides of the recording paper 110.

The paper discharge path 135 is provided with color sensors 200 fordetecting a patch image on the recording paper 110. Four color sensors200 are arranged in a direction orthogonal to the conveyance directionof the recording paper 110 and thus can detect four-column patch images.When a color detection command is received from an operation unit 180,the engine control unit 102 executes a maximum density adjustment, agradation adjustment, and multi-color correction processing.

A conveyance path switching flapper 134 serves as a leading member forleading the recording paper 110 to the paper discharge path 139connected to the outside. The recording paper 110 conveyed through thepaper discharge path 139 is discharged to the outside of the imageforming apparatus 100.

FIG. 2 illustrates a configuration of a color sensor 200. Each of thecolor sensors 200 includes a white light-emitting diode (LED) 201, adiffraction grating 202, a line sensor 203, a calculation unit 204, anda memory 205 therein. The white LED 201 is a light emission element forirradiating light onto a patch image 220 on the recording paper 110. Thediffraction grating 202 divides light reflected from the patch image 220by wavelength. The line sensor 203 is a photo-detection elementincluding the n number of light-sensitive elements for detecting lightdivided by wavelength by the diffraction grating 202. The calculationunit 204 performs various calculations based on a light intensity valueof each pixel detected by the line sensor 203.

A memory 205 stores various types of data used by the calculation unit204. The calculation unit 204 includes, for example, a spectralcalculation unit which performs a spectral calculation based on thelight intensity value and a Lab calculation unit which calculates a Labvalue. The calculation unit 204 may further include a lens 206 whichcondenses light irradiated from the white LED 201 onto the patch image220 on the recording paper 110 and condenses light reflected from thepatch image 220 onto the diffraction grating 202.

FIG. 3 is a block diagram illustrating a system configuration of theimage forming apparatus 100. The maximum density adjustment, thegradation adjustment, and the multi-color correction processing aredescribed below with reference to FIG. 3.

A printer controller 103 instructs an engine control unit 102 to outputa test chart to be used in the maximum density adjustment. At the time,the patch image 220 for adjusting the maximum density is formed on therecording paper 110 according to charged potential, exposure intensity,and developing bias set preliminary or set at the time of the lastmaximum density adjustment. Then, the engine control unit 102 instructsa color sensor control unit 302 to measure colors of the patch image220.

When the colors of the patch image 220 are measured by the color sensors200, a result of the color measurement is transmitted to a densityconversion unit 324 as spectral reflectance data. The density conversionunit 324 converts the spectral reflectance data into density data of thecolors of cyan (C), magenta (M), yellow (Y), and black (K) and transmitsthe converted density data to a maximum density correction unit 320.

The maximum density correction unit 320 calculates correction amountsfor the charged potential, the exposure intensity, and the developingbias such that the maximum density of the output image becomes a desiredvalue, and transmits the calculated correction amounts to the enginecontrol unit 102. The engine control unit 102 uses the receivedcorrection amounts for the charged potential, the exposure intensity,and the developing bias on and after the next image forming operation.According to the above operation, the maximum density of the outputimage is adjusted.

When the maximum density adjustment is completed, the printer controller103 instructs the engine control unit 102 to form a 16-gradation patchimage on the recording paper 110. Examples of an image signal of the16-gradation patch image may include 00H, 10H, 20H, 30H, 40H, 50H, 60H,70H, 80H, 90H, A0H, B0H, C0H, D0H, E0H, and FFH.

At the time, the 16-gradation patch image is formed on the recordingpaper 110 by using the correction amounts for the charged potential, theexposure intensity, and the developing bias calculated in the maximumdensity adjustment. When the 16-gradation patch image is formed on therecording paper 110, the engine control unit 102 instructs the colorsensor control unit 302 to measure the colors of the patch image 220.

When the colors of the patch image 220 are measured by the color sensors200, a result of the color measurement is transmitted to the densityconversion unit 324 as the spectral reflectance data. The densityconversion unit 324 converts the spectral reflectance data into densitydata of the colors of cyan (C), magenta (M), yellow (Y), and black (K),and transmits the converted density data to a density gradationcorrection unit 321. The density gradation correction unit 321calculates a correction amount for an exposure amount such that adesired gradation can be obtained. A look up table (LUT) generation unit322 generates a monochromatic gradation LUT and transmits themonochromatic gradation LUT to a LUT unit 323 as a signal value of eachof the colors of cyan (C), magenta (M), yellow (Y), and black (K).

Upon performing the multi-color correction processing, the image formingapparatus 100 generates a profile based on a detection result of thepatch image 220 including multiple colors, and converts an input imageby using the profile to form an output image thereof. An InternationalColor Consortium (ICC) profile which has recently been accepted in themarket is used here as an example of the profile for realizing anexcellent color reproducibility. The present exemplary embodiment,however, can also be applied to Color Rendering Dictionary (CRD)employed from PostScript Level 2 proposed by Adobe Systems Incorporated,and a color separation table with Adobe Photoshop, in addition to theICC profile.

When a customer engineer exchanges parts, or when a user executes a jobrequiring a color matching accuracy or desires to know a tint of a finaloutput in his design conceptual phase, the engineer or user operates theoperation unit 180 to instruct generation of a color profile.

The printer controller 103 performs the profile generation processing.The printer controller 103 includes a central processing unit (CPU)which reads out a program for executing a below-described flow chartfrom a storage unit 350 to run the program. For the sake of easyunderstanding of the processing performed by the printer controller 103,FIG. 3 illustrates an interior configuration of the printer controller103 in a block diagram.

When the operation unit 180 receives a profile generation command, aprofile generation unit 301 outputs a CMYK color chart 210 as anInternational Organization for Standardization (ISO) 12642 test form onthe engine control unit 102 without using the profile. The profilegeneration unit 301 transmits a color measurement command to a colorsensor control unit 302. The engine control unit 102 controls the imageforming apparatus 100 to cause the image forming apparatus 100 toexecute a charge processing, an expose processing, a developmentprocessing, a transfer processing, and a fixing processing. Accordingly,the ISO12642 test form is formed on the recording paper 110. The colorsensor control unit 302 controls the color sensors 200 so as to measurethe colors of the ISO12642 test form. The color sensors 200 output thespectral reflectance data resulting from a colorimetric measurement on aLab calculation unit 303 of the printer controller 103. The Labcalculation unit 303 converts the spectral reflectance data into L*a*b*data to output the L*a*b* data on the profile generation unit 301. TheLab calculation unit 303 may convert the spectral reflectance data intoa Commission Internationale de l'Eclairage (CIE) 1931 XYZ colorspecification system having a device-independent color space signal.

The profile generation unit 301 generates an output ICC profile based ona relationship between CMYK color signals output on the engine controlunit 102 and the L*a*b* data input from the Lab calculation unit 303.The profile generation unit 301 stores thus generated output ICC profilereplacing the output ICC profile currently stored in the output ICCprofile storage unit 305.

The ISO12642 test form includes patches of color signals of the colorsC, M, Y, and K covering a color reproduction range where a typical copymachine can output. Thus, the profile generation unit 301 creates acolor conversion table based on a relationship between a color signalvalue of each of the colors and the measured L*a*b* data value. Morespecifically, a conversion table for converting color signals of thecolors C, M, Y and K into the Lab value is generated. A reverseconversion table is generated based on the conversion table.

When the profile generation unit 301 receives a profile generationcommand from a host computer via an interface (I/F) 308, the profilegeneration unit 301 outputs the generated output ICC profile on the hostcomputer via the I/F 308. The host computer can execute the colorconversion corresponding to the ICC profile with an application program.

In the color conversion in a normal color output, an image signal whichis input from a scanner unit via the I/F 308 on the assumption of RGB(Red, Green, Blue) signal values and CMYK signal values in standardprinting colors such as JapanColor, is transmitted to an input ICCprofile storage unit 307 which receives input from external devices. Theinput ICC profile storage unit 307 converts the RGB signals into theL*a*b* data or the CMYK signals into the L*a*b* data according to theimage signal input via the I/F 308. The input ICC profile stored in theinput ICC profile storage unit 307 includes a plurality of LUTs (look uptables).

Examples of the LUTs include a one-dimensional LUT for controlling agamma value of the input signal, a multi-color LUT called as a directmapping, and a one-dimensional LUT for controlling the gamma value ofthus generated conversion data. The input image signal is converted froma color space dependent on a device into the L*a*b* data independentfrom the device with the LUTs.

The image signal converted into L*a*b* color space coordinates is inputinto a color management module (CMM) 306. The CMM 306 executes varioustypes of color conversions. For example, the CMM 306 executes a gamutconversion in which mapping of a mismatch is performed between a readingcolor space such as a scanner unit as an input device and an outputcolor reproduction range of the image forming apparatus 100 as an outputdevice. The CMM 306 further executes a color conversion for adjusting amismatch between a type of light source at the time of input and a typeof light source at the time of observing an output object (the mismatchis also referred to as a mismatch of a color temperature setting).

As described above, the CMM 306 converts the L*a*b* data into L′*a′*b′*data to output the converted data on an output ICC profile storage unit305. A profile generated according to the color measurement is stored inthe output ICC profile storage unit 305. Thus, the output ICC profilestorage unit 305 performs a color conversion of the L′*a′*b′* data byusing a newly generated ICC profile to further convert the resultingdata into the signals of the colors C, M, Y, and K dependent on anoutput device.

The LUT unit 323 corrects gradation of the signals of the colors C, M,Y, and K by means of the LUT set by the below-described LUT generationunit 322. The signals of the colors C, M, Y, and K of which gradation iscorrected are output to the engine control unit 102.

In FIG. 3, the CMM 306 is separated from an input ICC profile storageunit 307 and an output ICC profile storage unit 305. However, asillustrated in FIG. 4, the CMM 306 includes a module for performing acolor management. In other words, the CMM 306 performs a colorconversion by using an input profile (i.e., a print ICC profile 501) andan output profile (i.e., a printer ICC profile 502).

A thermochromism characteristic per color is described below. As amolecular structure forming a color material such as toner and inkvaries by heat, a reflection absorption characteristic of light variesand the chromaticity changes. As a result of a verification of a test,it is found that a trend of the chromaticity change differs betweencolor materials as illustrated in FIG. 5. A horizontal axis of the graphof FIG. 5 indicates a temperature variation of the patch image 220, anda vertical axis of the graph of FIG. 5 indicates a chromaticityvariation ΔE relative to a reference value at the temperature 15° C.

ΔE can be indicated by a three dimensional distance expressed in thefollowing equation between two points (L1, a1, b1) and (L2, a2, b2)within the L*a*b* color space established by CIE.

ΔE=√{square root over ((L1−L2)²+(a1−a2)²+(b1−b2)²)}{square root over((L1−L2)²+(a1−a2)²+(b1−b2)²)}{square root over((L1−L2)²+(a1−a2)²+(b1−b2)²)}

FIG. 5 illustrates a case of cyan (C) 100%, magenta (M) 100%, yellow (Y)100%, black (K) 100%, and white paper (W). As illustrated in FIG. 5, thevariation in a case of magenta is particularly great. The higher thetemperature of the patch image 220 becomes, the greater the chromaticityof the patch image 220 varies. As a result thereof, a deviation occursin the ICC profile to be generated.

As an index of color matching accuracy and color stability, the colormatching accuracy standard established by ISO 12647-7 (i.e., IT8.7/4(ISO 12642:1617 patch) [4.2.2]) defines that an average of ΔE is 4.0.The color reproducibility [4.2.3] as a standard of the color stabilitydefines that ΔE is equal to or less than 1.5 (i.e., ΔE≦1.5) in eachpatch. To satisfy the conditions, it is desired that detection accuracyfor ΔE of the color sensors 200 is equal to or less than 1.0 (i.e.,ΔE≦1.0). As illustrated in FIG. 5, to realize ΔE≦1.0 in all the colorsY, M, C, and K, a temperature of the patch image 220 is required to belowered to a value equal to or less than 34° C.

As described above, the chromaticity value (i.e., Lab value) varies withrespect to a temperature. On the other hand, as a result of a study bythe present applicant, it is found that a density value hardly varieseven while the temperature varies, i.e., there is no correlation betweenthe density value and the temperature. The result thereof is illustratedin FIG. 6.

The phenomenon that the chromaticity value varies but the density valuedoes not vary according to the temperature variation can be describedbased on differences in calculation methods upon calculation to obtainan area in which the spectral reflectance varies, a chromaticity value,and a density value. The above-described phenomenon is described belowby exemplifying the color of magenta (M) having a larger chromaticityvariation ΔE with respect to the temperature variation.

FIGS. 7A, 7B, and 7C illustrate spectral reflectance data in differenttemperatures when the patch image 220 in magenta is measured by thecolor sensors 200. FIG. 7A is an enlarged view of the entire wavelengtharea of a range between 400 nm and 700 nm. FIG. 7B is an enlarged viewof a wavelength area of a range between 550 nm and 650 nm. FIG. 7C is anenlarged view of a wavelength area of a range between 500 nm and 580 nm.

As illustrated in FIG. 5, in a case where a temperature of the patchimage 220 changes from 15° C. to 60° C., the chromaticity variation ΔEof magenta becomes about 2.0. The value of the chromaticity variation ΔEis obtained based on the variation of the spectral reflectance. FIG. 7Billustrates that the spectral reflectance varies according to thetemperature variation of the patch image 220. That is, the Labcalculation unit 303 calculates the chromaticity by using a spectralreflectance with respect to the entire wavelength area, so that thechromaticity value varies as the spectral reflectance varies.

On the other hand, as illustrated in FIG. 6, the density hardly varieswhile the temperature of the patch image 220 varies from 15° C. to 60°C. That is, the density conversion unit 324 calculates the density byusing the spectral reflectance with respect to a specific wavelengtharea. A variation of the spectral reflectance is not clearly illustratedin FIG. 7A. However, in FIG. 7B illustrating the enlarged wavelengtharea of a range between 550 nm and 650 nm, a state where the variationof the temperature of the patch image 220 causes the variation of thespectral reflectance is clearly illustrated. That is, since the Labcalculation unit 303 calculates the chromaticity by using the spectralreflectance with respect to the entire wavelength area, the chromaticityvalue varies as the spectral reflectance varies.

That is, the density conversion unit 324 calculates the density by usingthe spectral reflectance with respect to a specific wavelength area.More specifically, the density conversion unit 324 converts the spectralreflectance data of the colors of cyan, magenta, and yellow into densitydata by using a filter illustrated in FIG. 8A. The density conversionunit 324 converts the spectral reflectance data of the color of blackinto density data by using a visibility spectral characteristic asillustrated in FIG. 8B.

It is understood that the spectral reflectance hardly varies in thewavelength area in FIG. 7C. The area of FIG. 7C corresponds to an areahaving a sensitivity characteristic of a color of green among thewavelength area of the horizontal axis illustrated in FIG. 8A. Thedensity value of magenta is calculated by using the sensitivitycharacteristic of the color of green as a complementary color.Therefore, in this area, the spectral reflectance hardly varies even asthe temperature varies, so that the density value hardly varies.

As described above, while the chromaticity of the patch image 220 variesaccording to the temperature variation, the density of the patch image220 hardly varies according to the temperature variation. In the presentexemplary embodiment, upon multi-color correction (i.e., upon generationof the ICC profile), the color measurement is to be performed by thecolor sensors 200 after allowing heat on the recording paper 110 heatedby a fixing device to dissipate. However, when adjusting the maximumdensity or the gradation, the color measurement is to be performed bythe color sensors 200 without allowing heat on the recording paper 110to dissipate.

FIG. 9 is a flow chart illustrating an operation of the image formingapparatus 100. The flow chart is executed by the printer controller 103.In step S901, the printer controller 103 determines whether an imageforming request is received from the operation unit 180 or whether theimage forming request is received from the host computer via the I/F308.

In step S902, if no image forming request is received (NO in step S902),the printer controller 103 determines whether a multi-color correctioncommand is received from the operation unit 180. If the multi-colorcorrection command is received (YES in step S902), then in step S903,the printer controller 103 performs the maximum density adjustment in amanner as illustrated in FIG. 10. In step S904, the printer controller103 further performs the gradation adjustment in a manner as illustratedin FIG. 11. In step S905, the printer controller 103 still furtherperforms multi-color correction processing in a manner as illustrated inFIG. 12. If, in step S902, no multi-color correction command is received(NO in step S902), the processing returns to step S901. As describedabove, the maximum density adjustment and the gradation adjustment areperformed before the multi-color correction processing in order toachieve highly accurate multi-color correction processing.

If, in step S901, the printer controller 103 determines that an imageforming request is received (YES in step S901), then in step S906, theprinter controller 103 causes the storage unit 113 to feed the recordingpaper 110, to form a toner image on the recording paper 110 in stepS907. In step S908, the printer controller 103 determines whether animage formation is completed for all the pages. If the image formationis completed for all the pages (YES in step S908), the processingreturns to step S901. If the image formation is not completed for allthe pages (NO in step S908), the processing returns to step S906 to formthe image formation for the next page.

FIG. 10 is a flow chart illustrating an operation of the maximum densityadjustment. The flow chart is executed by the printer controller 103.The image forming apparatus 100 is controlled by the engine control unit102 according to an instruction from the printer controller 103.

In step S1001, the printer controller 103 causes the storage unit 113 tofeed the recording paper 110, to form a patch image 220 on the recordingpaper 110 for the maximum density adjustment in step S1002.Subsequently, in step S1003, the printer controller 103 causes the colorsensors 200 to measure the patch image 220 when the recording paper 110arrives at the color sensors 200.

In step S1004, the printer controller 103 causes the density conversionunit 324 to convert the spectral reflectance data output from the colorsensors 200 into density data of the colors of C, M, Y, and K. In stepS1005, the printer controller 103 calculates the correction amounts forthe charged potential, the exposure intensity, and the developing biasbased on the converted density data. The correction amounts calculatedhere are stored in the storage unit 350 to be used thereby.

FIG. 11 is a flow chart illustrating an operation of the gradationadjustment. The flow chart is executed by the printer controller 103.The image forming apparatus 100 is controlled by the engine control unit102 according to a command from the printer controller 103.

In step S1101, the printer controller 103 causes the storage unit 113 tofeed a recording paper 110, to form a patch image (i.e., a 16-gradationpatch image) on the recording paper 110 for the gradation adjustment instep S1102. In step S1103, the printer controller 103 causes the colorsensors 200 to measure the patch image 220 when the recording paper 110arrives at the color sensors 200.

In step S1104, the printer controller 103 causes the density conversionunit 324 to convert the spectral reflectance data output from the colorsensors 200 into density data of the colors Y, M, C, and K. In stepS1105, the printer controller 103 calculates the correction amount forthe exposure intensity based on thus converted density data, to generatea LUT for correcting the gradation in step S1105. The LUT calculatedhere is set in the LUT unit 323 to be used thereby.

FIG. 12 is a flow chart illustrating an operation of multi-colorcorrection processing. The flow chart is executed by the printercontroller 103. The image forming apparatus 100 is controlled by theengine control unit 102 according to a command from the printercontroller 103.

In step S1201, the printer controller 103 causes the storage unit 113 tofeed a recording paper 110, to form a patch image 220 on the recordingpaper 110 for the multi-color correction processing in step S1202. Instep S1203, the printer controller 103 stops conveying the recordingpaper 110 by controlling a conveyance roller driving motor 311 when thetrailing edge of the recording paper 110 is detected by a reversalsensor 137. The printer controller 103 stops the recording paper 110 ina reversal unit 136 for a predetermined period of time (e.g., 40 sec. inthe present exemplary embodiment) to allow heat on the patch image 220on the recording paper 110 to dissipate. Accordingly, a chromaticityvariation caused due to an adverse effect of the thermochromism can bedecreased.

As illustrated in FIG. 5, the temperature of the patch image 220 isrequired to be lowered at a temperature equal to or less than 34° C. inorder to realize ΔE values equal to or less than 1.0 (i.e., ΔE≦1.0) inall the colors Y, M, C, and K. A period of time required for this heatdissipation is set to 40 sec. in the present exemplary embodiment. Whilethe recording paper 110 is stopped for 40 sec., the temperature of thepatch image 220 can be decreased to a temperature equal to or less than34° C., even in a case where both of a first fixing heater 312 providedon the primary fixing device 150 and a second fixing heater 313 providedon the secondary fixing device 160 are used.

After the stop time of 40 sec. has elapsed, in step S1204, the printercontroller 103 controls the conveyance roller driving motor 311 torestart conveying the recording paper 110. At the time, the printercontroller 103 conveys the recording paper 110 toward the color sensors200 in the opposite direction.

In step S1205, when the recording paper 110 arrives at the color sensors200, the printer controller 103 causes the color sensors 200 to measurethe patch image 220 on the recording paper 110. The printer controller103 calculates the chromaticity data (L*a*b*) based on the spectralreflectance data output from the color sensors 200 by using the Labcalculation unit 303. In step S1206, the printer controller 103generates the ICC profile based on the chromaticity data (L*a*b*)according to the above-described processing, to store the resulting ICCprofile in the output ICC profile storage unit 305 in step S1207.

As described above, in the present exemplary embodiment, the colors ofthe patch image 220 are measured by the color sensors 200 withoutproviding the stop time for stopping the recording paper 110 in themaximum density adjustment and the gradation adjustment among theoperations of the color correction processing. On the other hand, asillustrated in step S1203, in the multi-color correction processing, therecording paper 110 having passed through the fixing device is stoppedfor a predetermined period of time to allow heat on the recording paper110 to dissipate as much as possible and thereafter to cause the colorsensors 200 to measure the colors of the patch image 220.

In other words, in the present exemplary embodiment, a period of time,for calculating the chromaticity value by the Lab calculation unit 303,from when the recording paper 110 passes through the fixing device untilthe color measurement is performed by the color sensors 200, iscontrolled to be longer than the period of time for calculating thedensity value by the density conversion unit 324. Accordingly, in thepresent exemplary embodiment, the chromaticity variation caused due tothe adverse effect of the thermochromism can be decreased in themulti-color correction processing, thereby enabling a highly accuratedetection of the chromaticity of the patch image 220.

In the above description, the recording paper 110 is temporarily stoppedbefore the color measurement is performed by the color sensors 200 afterthe recording paper 110 passes through the fixing device, therebyallowing heat on the recording paper 110 to dissipate. Instead of thetemporal stopping of the recording paper 110, a conveyance speed toconvey the recording paper 110 may be decreased.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

What is claimed is:
 1. An image forming apparatus comprising: aconveyance unit configured to convey a recording paper along aconveyance path; an image forming unit configured to form a measurementimage on the recording paper by using at least one of color materials; afixing unit configured to heat the measurement image on the recordingpaper to fix the measurement image onto the recording paper; ameasurement unit configured to measure the measurement image fixed ontothe recording paper downstream of the fixing unit in the conveyancepath; a first generation unit configured to generate color data based ona measurement result of the measurement unit; a second generation unitconfigured to generate density data based on a measurement result of themeasurement unit; and a controller configured to control the conveyanceunit so that a first period becomes longer than a second period, whereinthe first period is a period from a time point that the measurementimage on the recording paper passes the fixing unit until a time pointthat the measurement unit completes the measurement of the measurementimage, in a case where the first generation unit generates the colordata, and wherein the second period is a period from a time point thatthe measurement image on the recording paper passes the fixing unituntil a time point that the measurement unit completes the measurementof the measurement image, in a case where the second generation unitgenerates the density data.
 2. The image forming apparatus according toclaim 1, wherein the controller controls the conveyance unit so that aconveyance of the recording paper on which the measurement image isfixed stops for a predetermined time period during the period from thetime point that the measurement image on the recording paper passes thefixing unit until the time point that the measurement unit completes themeasurement of the measurement image, in a case where the firstgeneration unit generates the color data.
 3. The image forming apparatusaccording to claim 2, wherein, after the recording paper is stopped forthe predetermined time period at a reversal unit, the conveyance unitconveys the recording paper to a measurement position of the measurementunit by reversing a conveyance direction.
 4. The image forming apparatusaccording to claim 3, wherein, in a case where the first generation unitgenerates the color data, the measurement unit measures the measurementimage on the recording paper that is conveyed from the reversal unit. 5.The image forming apparatus according to claim 1, wherein the controllercontrols a conveyance speed for the conveyance unit to convey therecording paper, and wherein the conveyance speed for the conveyanceunit to convey the recording paper in a case where the first generationunit generates the color data, is slower than the conveyance speed forthe conveyance unit to convey the recording paper in a case where thesecond generation unit generates the density data.
 6. The image formingapparatus according to claim 1, wherein the measurement unit comprisesan irradiating unit configured to irradiate light onto the measurementimage and a light receiving unit configured to receive reflection lightfrom the measurement image, the measurement unit is configured togenerate spectral data of the measurement image based on a lightreceiving result of the light receiving unit, the first generation unitgenerates the color data based on the spectral data output by themeasurement unit, and the second generation unit generates the densitydata based on the spectral data output by the measurement unit.
 7. Theimage forming apparatus according to claim 6, wherein the firstgeneration unit generates the color data based on a spectral data in afirst wavelength area included in the spectral data output by themeasurement unit, the second generation unit generates the density databased on a spectral data in a second wavelength area included in thespectral data output by the measurement unit, and the second wavelengtharea is narrower than the first wavelength area.
 8. The image formingapparatus according to claim 1, wherein the image forming unit forms acolor image by using first colorant and second colorant, in a case wherethe first generation unit generates the color data, the image formingunit forms the measurement image by mixing the first colorant and thesecond colorant, and in a case where the second generation unitgenerates the density data, the image forming unit forms the measurementimage without mixing the first colorant and the second colorant.